The present invention relates to a method and apparatus for generating electric power by combusting waste products. More particularly, it relates to a method and apparatus for generating electric power at high efficiency from a high-temperature and high-pressure steam which is produced by using the heat of exhaust gas generated when various waste products are combusted, while avoiding heat exchanger problems caused by corrosion due to high-temperature molten salt.
It is expected that in the 21st century, the treatment of various waste products will change greatly from simple incineration to dioxin-free recycling system which can recover energy at high efficiency. Because more efforts will be directed to sorting waste products, the recycling technology according to the type of waste product will be desired. For example, a gasification and melt combustion system which is capable of simultaneously coping with the control of dioxin and melting of ash by using combustion heat generated when waste products are combusted is becoming the main option for treating general waste products. A chemical recycling technology which is capable of converting plastic-based waste products into raw material for chemicals is becoming the main option for treating plastic-based waste products. Highly efficient power generation by combustion of waste products will require a power generation efficiency of 30% or higher.
Present thermal recycling systems for generating electric power by using thermal energy produced by the combustion of wastes is generally carried out by recovering the combustion heat of waste products in the form of steam using a waste heat boiler and supplying the steam to a steam turbine to generate electric power by a generator driven by the steam turbine.
One example of a conventional power generation system based on waste incineration is shown in FIG. 4 of the accompanying drawings. As shown in FIG. 4, waste products are combusted in an incinerator or a gasification and melt combustion furnace 11, and the heat of generated exhaust gas is recovered by a waste heat boiler 13, which produces superheated steam. The superheated steam is supplied to a steam turbine 15 to which a generator is directly coupled, thus generating electric power. The generated electric power is consumed in the waste incineration facility, and is sold to the power company. The exhaust gas that has passed through the waste heat boiler 13 flows through a preheater 16 (such as an economizer) and a bag filter 17, and then is discharged through a stack into the atmosphere as low-temperature clean gas.
The efficiency of power generation in the conventional steam-turbine power generation system greatly depends on the temperature of the superheated steam supplied to the steam turbine. The efficiency of power generation is remarkably increased as the temperature of the superheated steam is increased. In the conventional practical system for power generating by waste incineration, the temperature of the superheated steam has been about 400xc2x0 C. at maximum for the following reason, and the efficiency of power generation has been about 20% at most.
Heretofore, the temperature of the superheated steam has not been increased beyond about 400xc2x0 C. because of corrosion caused by corrosive gas such as hydrogen chloride produced during waste combustion and corrosion caused by a high-temperature molten salt. As for the waste heat boiler, since saturated steam having a relatively low temperature, e.g. about 310xc2x0 C. flows through water pipes even under the pressure of about 100 kg/cm2, corrosion of the water pipes can be prevented even if metal pipes are used. However, in the case of superheated steam, since the temperature of superheated steam is a high temperature of 400xc2x0 C. or higher, the surface of the heat transfer metal pipes will be damaged by corrosion caused by corrosive component such as high-temperature molten salt.
The mechanism of corrosion is complicated and the corrosive reaction is affected by a combination of various factors. The most important factor for the corrosion of the heat transfer tube is whether the heat transfer tube is exposed to an environment containing molten salt of NaC1 and/or KC1, rather than the concentration of HC1. Under this environment, salts are melted, adhere to the heat transfer tubes and accelerate the corrosion thereof.
FIG. 5 shows different forms of corrosion depending on the temperature of exhaust gas (represented on the horizontal axis) produced by combustion of wastes and the surface temperature of a heat transfer tube (represented on the vertical axis), deduced from the long experience of the inventors and a corrosion test using a municipal waste incinerator. As shown in FIG. 5, there are four forms, i.e., xe2x80x9cintense corrosion regionxe2x80x9d, xe2x80x9ccorrosion progress regionxe2x80x9d, xe2x80x9ccorrosion retardation regionxe2x80x9d, and xe2x80x9ccorrosion-free regionxe2x80x9d. When the temperature of superheated steam is 400xc2x0 C., the surface temperature of the heat transfer tube is about 430xc2x0 C. which is higher than the temperature of superheated steam by about 30xc2x0 C. At this temperature, the temperature of exhaust gas of 600xc2x0 C. or thereabout is considered to be a boundary temperature separating the xe2x80x9ccorrosion progress regionxe2x80x9d and the xe2x80x9ccorrosion-free regionxe2x80x9d from each other. This coincides with the fact that when the temperature of exhaust gas entering a boiler bank where steam pipes are gathered closely is 600xc2x0 C. or higher in the waste heat boiler of a municipal waste incinerator, salts adhere to heat transfer tubes causing exhaust gas passages to be clogged. Therefore, the boundary temperature between when salts are melted or solidified is considered to be about 600xc2x0 C. This temperature corresponds to the melting point of complex salts. The melting point of NaC1 is 800xc2x0 C., and the melting point of KC1 is 776xc2x0 C. However, salts turn into complex salts after being melted, and their melting point is lowered, e.g., to the range of 550 to 650xc2x0 C. This melting point varies with the properties of the waste products which differ from place to place. For example, this boundary temperature may be lower than 600xc2x0 C. in local cities close to seashores, because salts are present at higher concentration in the waste products. Even if the temperature of exhaust gas is in the range of 500 to 600xc2x0 C., when the surface temperature of the heat transfer tube is equal to or higher than about 430xc2x0 C., its environment belongs to the xe2x80x9ccorrosion retardation regionxe2x80x9d, and the heat transfer tube suffers slight corrosion, less intense than molten-salt corrosion. Accordingly, the selection of the superheater tube material for use at the boundary temperature is of importance. Inasmuch as the surface temperature of the heat transfer tube is about 30xc2x0 C. higher than the temperature of the superheated steam, the temperature of the superheated steam, which is about 400xc2x0 C., is considered to be an allowable temperature limit for preventing corrosion. If the temperature of the superheated steam is 400xc2x0 C., the pressure of the steam is suppressed to about 3.9 MPa on account of the problem of a turbine drain attack. Hence, the efficiency of power generation is only about 20% in the waste incinerating power generating system.
In order to obtain superheated steam at a temperature of 400xc2x0 C. or higher while avoiding the xe2x80x9ccorrosion progress regionxe2x80x9d, it is necessary to install superheated steam pipes in the condition that the temperature of exhaust gas is in the range, of 500 to 600xc2x0 C. Such a method is disadvantageous, however, in that since the temperature difference between the exhaust gas and the superheated steam is small, in order to obtain a desired amount of heat transfer, the required heat transfer surface is too large, resulting in a large-size heat recovery facility.
On the other hand, attempts have heretofore been made to develop corrosion-resistant metallic materials for the purpose of higher power generation efficiency by increasing the steam temperature without corrosion of the heat transfer tube. However, such attempts at material development are technically and economically so burdensome that practically no satisfactory results have been obtained yet. Another effort is to develop an RDF power generation system in which waste products are added with lime and converted into solid fuel for dechlorination and desulfuiization. Although the RDF power generation system is capable of reducing HC1 in the exhaust gas, it fails to reduce the molten-salt corrosion. From FIG. 5, in the case where the superheated steam having a temperature of 500xc2x0 C. is obtained by the exhaust gas having a temperature of 800xc2x0 C. or higher, the surface temperature of the heat transfer tube is about 530xc2x0 C., so that the heat transfer tube is exposed to the xe2x80x9cintense corrosion regionxe2x80x9d.
There has been proposed a system, the so-called advanced waste power generating system, in which electric power is generated by a gas turbine and the steam from a waste heat boiler is reheated by the exhaust gas from the gas turbine to thereby increase the efficiency of power generation in a steam-turbine. However, the proposed system is problematic in that it consumes a lot of another high-quality fuel other than waste products and is not economical. Another proposed system is such a system in which the steam from a waste heat boiler is reheated by combusting high-quality fuel to increase the efficiency of power generation in a steam turbine. This system is also economically problematic because it consumes a lot of another high-quality fuel other than waste products.
It is therefore an object of the present invention to provide a method and apparatus for generating electric power by combusting waste products which is capable of increasing the temperature of superheated steam for increased power generation efficiency without the problem of corrosion of a heat exchanger caused by corrosive components such as high-temperature molten salts contained in combustion gas of waste products.
According to one aspect of the present invention, there is provided a method for generating electric power by combusting wastes, the method comprising: combusting waste products to generate exhaust gas having a high temperature; introducing the exhaust gas into a heat exchanger to heat an intermediate gas by heat exchange; heating superheated steam by utilizing the heated intermediate gas as a heat source; and supplying the heated superheated steam to a steam turbine coupled to a generator to generate electric power.
According to another aspect of the present invention, there is provided an apparatus for generating electric power by combusting waste products, the apparatus comprising: a combustor for combusting wastes to generate exhaust gas having a high temperature; a heat exchanger for heating an intermediate gas by heat exchange between the exhaust gas from the combustor and the intermediate gas; a heater for heating superheated steam by heat exchange between the heated intermediate gas and the superheated steam; and a steam turbine coupled to a generator, the heated superheated steam being supplied to the steam turbine to generate electric power.
According to the present invention, exhaust gas produced by waste combustion is introduced into the heat exchanger where an intermediate gas such as air flowing through heat transfer tubes is heated by heat exchange. Then superheated steam obtained by a waste heat boiler or the like is heated by utilizing the heated intermediate gas as a heat source, and the superheated steam heated by the intermediate gas is supplied to the steam turbine coupled to the generator to generate electric power. That is, the exhaust gas is not introduced into the waste heat boiler directly to heat superheated steam, but the exhaust gas is introduced into the heat exchanger to heat an intermediate gas such as air and the superheated steam is heated by the heated intermediate gas. The heat exchanger which heats the intermediate gas such as air by exhaust gas is not required to be under high pressure. Hence, it is unnecessary for the heat exchanger to use materials which are laid down by standards such as a boiler-structure standard or a technical standard for the power plant. Thus, ceramic material can be used as material for the heat exchanger. If ceramic material is used for the heat transfer tubes in the heat exchanger, they are hardly susceptible to corrosion caused by molten salt. Even in the case where a heat exchanger made of heat resisting cast steel or metal is used, the heat transfer tubes which are exposed to exhaust gas can be used under environment of the xe2x80x9ccorrosion retardation regionxe2x80x9d shown in FIG. 5, thus hardly causing the problem of molten salt corrosion. Specifically, although the heat resisting cast iron has been known as material having corrosion resistance, it has not been able to be used as a steam pipe through standards in the conventional method. However, the heat resisting cast iron will be able to be used in the present invention because it is used not as a steam pipe but as a heat transfer pipe between the exhaust gas and intermediate gas such as air.
According to the above-mentioned present invention, the high temperature exhaust gas heats intermediate gas such as air which flows through the heat transfer tubes. Corrosive components contained in the exhaust gas hardly cause high-temperature molten-salt corrosion of the tubes when the surface temperature of the tubes is 700xc2x0 C. or higher. When the surface temperature of the heat transfer tubes is about 700xc2x0 C. or higher, the corrosion of the tubes is small because the surrounded environment is in the xe2x80x9ccorrosion retardation regionxe2x80x9d shown in FIG. 5. Consequently, even if the heat transfer tubes in the high-temperature heat exchanger are made of metal, gas such as air flowing through the tubes can be heated to a high temperature of about 700xc2x0 C., for example. It is possible to easily generate superheated steam having a temperature of about 500xc2x0 C. by reheating the superheated steam which has been heated to about 400xc2x0 C. in a waste heat boiler with the intermediate gas such as air having a temperature of about 700xc2x0 C. Since the superheated steam is reheated not by the exhaust gas but by high-temperature intermediate gas such as air, the problem of high-temperature molten-salt corrosion does not occur. Because the high-temperature intermediate air is used as a heat source instead of exhaust gas, lowering of heat transfer coefficient due to adhesion of dust to the heat transfer tubes does not occur. Thus, the superheated steam heater may be relatively downsized. The superheated steam thus reheated is supplied to the steam turbine coupled to a generator, which can generate electric power with an efficiency of 30% or higher.
FIG. 6 shows corrosion rates of heat transfer tubes in a conventional system for heating superheated steam directly with exhaust gas and a system for heating superheated steam indirectly with exhaust gas according to the present invention. With the conventional system as shown by the curve A in FIG. 6, since the film heat transfer coefficient at the steam side is large, the surface temperature of the tubes is close to the steam temperature, so that the tubes are exposed to the xe2x80x9cintense corrosion regionxe2x80x9d. With the system according to the present invention as shown by the curve B in FIG. 6, intermediate gas such as air instead of steam is heated by the exhaust gas. In case of air, since the film heat transfer coefficient at the air side is much smaller than that at the steam side, the surface temperature of the tubes is close to the exhaust gas temperature, so that the tubes are exposed to the xe2x80x9ccorrosion retardation regionxe2x80x9d shown in FIG. 5. Inasmuch as the system for heating the superheated steam indirectly with the exhaust gas can avoid high-temperature molten-salt corrosion, the tubes which are made of an existing metallic material can be used.
Next, the difference in the surface temperature of the heat transfer tube which is caused by the difference of the film heat transfer coefficient will be described.
The surface temperature of the heat transfer tube Tw is calculated by the following equation.
Tw=Txe2x88x92{hio/(hio+ho)}xc3x97(Txe2x88x92t)
where T=temperature of exhaust gas
t=temperature of intermediate gas which receives heat
ho=film heat transfer coefficient of exhaust gas
hio=film heat transfer coefficient of fluid gas which receives heat
1) If superheated steam is used as heat receiving fluid
Assuming that T=1200xc2x0 C., t=500xc2x0 C., ho≈100 kcal/m2hxc2x0 C., and hio≈2000 kcal/m2hxc2x0 C. (very large value in case of the superheated steam), Tw=1200xe2x88x92{2000/(2000+100)}xc3x97(1200xe2x88x92500)=533xc2x0 C.
Thus, the heat transfer tubes are exposed to the xe2x80x9cintense corrosion regionxe2x80x9d.
2) If intermediate air is used as heat receiving fluid
Assuming that T=1200xc2x0 C., t=500xc2x0 C., ho≈100 kcal/m2hxc2x0 C., and hio≈200 kcal/M2hxc2x0 C. (small value in case of gas such as air), Tw=1200xe2x88x92{200/(200+100)}xc3x97(1200xe2x88x92500)=733xc2x0 C.
Thus, the intense corrosion of the heat transfer tubes is avoidable.
Further, since low-pressure air can be used as the heating medium, the tubes may be made of ceramic material. Whether the tubes are to be made of metallic material or ceramic material is determined depending on conditions in which they are used. For example, if the surface temperature of the tubes is 800xc2x0 C. or higher, the tubes are preferably made of ceramic material or heat-resistant cast steel.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.